Circuit arrangement for the reliable switching of electrial circuits

ABSTRACT

A circuit arrangement for the reliable switching of electrical circuits contains two series paths, two switching elements being arranged in parallel with one another in one of the series paths, the switching inputs of said switching elements being connected to the input point of the series path and the switching outputs of said switching elements being connected to the input side of a respective winding of a transformer.

For interrupting and making electrical circuits, use is made both ofmechanical contacts, sliding contacts such as in the case of currentcollectors on rail vehicles or commutators in electric motors, fusiblelinks, and of semiconductor switches such as transistors, thyristors andsemiconductor relays.

During interruption of the electrical circuits, all these switchingelements are exposed to a high self-induced voltage as a result of therapid reduction of the energy stored inductively in the entireelectrical circuit.

Said self-induced voltage heats and destroys semiconductor switchingelements and protective circuits, causes material migration and weldingat contact areas and can prevent the breaking of the electrical circuitas a result of arcing between contact areas.

During making of an electrical circuit, the capacitance present in thecircuit has to be charged rapidly, which momentarily leads to a largeswitch-on current.

Said switch-on current brings about material migrations at contact areasthat have not yet completely closed, and can destroy semiconductorswitches through local thermal overloading.

During the transition of the switching elements from the conducting tothe blocking state and from the blocking to the conducting state, apower loss is produced at the switching elements due to the simultaneouspresence of current and voltage, said power loss being referred to asswitching power.

In the case of frequent switching operations, this switching power leadsto the heating of the switching elements and of adjacent components, andthereby jeopardizes the reliable operation of entire apparatuses andinstallation.

In order to protect the switching elements from the harmful effects ofthe self-induced voltage, use is made of RC circuits, but the latter areheated greatly in the event of high switching frequency.

Diode circuits, also known as freewheeling diodes, protect the switchingelements from self-induced voltage only after a response time, butcannot be used with AC voltage, and cause a power loss during eachswitching operation, which limits the efficiency of frequently switchingcircuit arrangements such as voltage converters or switched-mode powersupplies and leads to the heating and damage thereof.

Furthermore, varistor circuits are known, which protect the switch fromparticularly high self-induced voltages. However, said varistors arerapidly heated and are therefore unsuitable in the event of highswitching frequency and high voltage and also for precise limiting ofthe overvoltages to low values, for the protection of semiconductorcomponents.

It is also known that the self-induced voltage and heating of theswitching element during interruption of the electrical circuit can beeffectively limited by means of capacitor connected in parallel with theload or else in parallel with the switching element. However, thiscircuit has the disadvantage that, during the closing of the switchingelement, the capacitor would have to be short-circuited or abruptlycharged, which causes very high switch-on currents, high switchinglosses and severe wear of the switching elements, so that thecapacitance of the capacitor remains limited to a very small value andthe effectiveness thereof is thus greatly restricted.

Taking this as a departure point, it is an object of the invention tospecify a circuit arrangement which enables the reliable switching ofelectrical circuits.

This object is achieved according to the invention by means of thecircuit arrangement having the features of claim 1.

The circuit arrangement according to the invention prevents theoccurrence of high self-induced voltage by means of a capacitor whichmomentarily accepts the current from the electrical load circuit to bebroken and, by means of its discharge operation, prevents a rapid riseof the voltage across the windings of the transformer and the openingswitching elements connected in series therewith.

The—according to the invention—reliable switch-off operation ofcurrent-carrying switching elements is in this case achieved by avoidingvoltage spikes, power loss and heating.

Avoiding power loss during the switch-off operation according to theinvention also prevents the production of arcs in the case ofelectromechanical switching elements and fusible links and thus enablesthe latter to be reliably switched off according to the invention.

If large quantities of energy are present in the electrical loadcircuit, the circuit arrangement according to the invention may beconfigured such that the load is short-circuited after disconnectionfrom the voltage source and the energy is held in the electrical loadcircuit.

During making of an electrical circuit, the load is connected to thevoltage source by the circuit arrangement according to the invention viaa transformer winding acting as an inductance, which brings about a slowcontrolled current rise and a slow controlled charging of thecapacitance in the electrical load circuit. The slow controlled currentrise and the small power loss in the switching element when the loadcurrent is switched on via an inductance enables an—according to theinvention—reliable closing operation of the switching elements.

The voltage at the transformer winding is transformed to a secondwinding, which, when the capacitance in the electrical load circuit ischarged to the voltage of the voltage source, brings a second switchingelement into a voltageless state in which it can be reliably closedaccording to the invention, with little power loss.

Since the circuit arrangement according to the invention enables thereliable switching operations substantially by avoiding power loss, theinvention can make a significant contribution to miniaturization andreduction of costs for frequently switching apparatuses such as DCvoltage converters, switched-mode power supplies, motor drives, since itpermits significantly higher switching frequencies.

Reducing the power loss during switching operations also makes animportant contribution to environmental protection.

Since the circuit arrangement according to the invention uses only onetransformer and a usually very small capacitor for limiting damagingvoltages and damaging rapid current rises, the circuit arrangement canbe used for DC voltages and sinusoidal or rectangular AC voltages.

Exemplary embodiments of the invention are illustrated in the drawingand are described in more detail below.

In the figures:

FIG. 1 shows a first exemplary embodiment of the circuit arrangementaccording to the invention,

FIG. 2 shows an example of alternating current direction in the load,

FIG. 3 shows an example with transistors and control device 33,

FIG. 4 shows an example of the operation of the circuit arrangementaccording to the invention from two voltage sources,

FIG. 5 shows an example of the operation of the circuit arrangementaccording to the invention from one AC voltage source formed from atransformer winding with a center tap, and

FIG. 6 shows an exemplary application of the circuit arrangementaccording to the invention.

The circuit arrangement according to the invention has two series paths1 and 2, which are located in the lead to the load 3 and contain twoconnecting points 4 and 5 for the connection of the voltage source, andalso two connecting points 6 and 7 for the connection of the load 3.

The series path 1 is subdivided into two further series paths containingin each case a winding 8 and 9 of the transformer 10 and also in eachcase a switching element 11 and 12, respectively.

Downstream of the transformer 10, the two windings 8 and 9 are connectedto one another and to the connecting point 6.

A capacitor 13 is connected in parallel with the load between the seriespaths 1 and 2. The capacitor 13 may be chosen to be very large in thecase of slowly switching circuit arrangements or high load currents,and, in the case of rapidly switching circuit arrangements, be so smallthat the line capacitance between 1 and 2 suffices for obtaining thedesired protective effect.

The load current 14 from series path 1 is divided into the componentcurrents 15 and 16 such that the magnetic field strengths 17 and 18thereof in the core of the transformer 10 act oppositely to one anotherand mutually compensate for one another.

If the two component currents 15 and 16 in the windings 8 and 9 are ofthe same magnitude, the load current 14 cannot store energy in thetransformer core.

If the component current 15 is interrupted by the switching element 11,then its compensating field strength 17 is omitted, as a result of whichthe remaining current-carrying winding 9 takes effect as an inductanceand momentarily interrupts the remaining component current 16 since thetransformer core is not yet magnetized. Therefore, immediately after theinterruption of the component current 15, the load current 14 is drawncompletely from the capacitor 13, so that the remaining, secondswitching element 12, according to the invention, can be reliably openedin a virtually currentless and voltageless state with little power loss.

The capacitor 13 is discharged by the load current 14 after the openingof switching element 11, as a result of which the capacitor voltagefalls, and a voltage is produced across the transformer winding 9, whichis transformed to the transformer winding 8. The voltage in transformerwinding 8, together with the voltage across capacitor 13, has the effectthat the switching element 11, according to the invention, can openreliably in a virtually voltageless state, with very little power loss.

In the simplest case, the switching elements 11 and 12 may comprise aswitching contact, a fusible link or a transistor, which undergotransition to the nonconducting state simultaneously, or with a shortdelay, when the electrical circuit is interrupted. The switchingelements 11 and 12 may also be formed as changeover contacts, push-pullor CMOS transistor stages which, after the disconnection of the seriespath 1 from the connecting point of the voltage source 4, establish aconnection to the series path 2 in order to conduct away the loadcurrent 14 and to hold the energy stored in the load 3 in the electricalload circuit.

In order to conduct away the energy stored in the electrical circuit,the switching elements 11 and 12 may also be provided with diodes whichconduct away the load current 14 after the disconnection from thevoltage source to the series path 2.

A further possibility is to measure the voltage across the switchingelements by means of a voltage measuring device and to establish theconnection to the series path 2 with a controllable switching elementwhen the measured voltage has become zero, thereby achievingan—according to the invention —reliable, low-loss closing operation in avoltageless state.

The load can be connected to the voltage source again by only one orboth switching elements 11 and 12 interrupting the connection to theseries path 2, and then only one switching element connecting thecorresponding transformer winding to the series path 1, so that thecapacitor 13 is charged via the winding inductance of the transformer10. In this case, according to the invention, the winding inductanceprevents a rapid rise of the charging current and thus enables,according to the invention, the reliable, low-loss closing of theswitching element in a virtually currentless state. If the capacitor 13is completely charged, the second switching element may likewiseestablish the connection to the voltage source in low-loss fashion in avirtually voltageless state reliably, according to the invention,whereupon the component currents 15 and 16 in the windings of thetransformer match one another.

A further embodiment of the circuit according to the invention consistsin the fact that the load 3 is connected to the series path 1 or 2 via acapacitor 19 connected in series. This results in an alternating currentdirection in the load, which permits the capacitor 13 to be charged tothe voltage of the voltage source by the load current in the event ofswitch-on. The two switching elements 11 and 12 can then establish theconnection to the voltage source in a currentless and voltageless statewithout energy being fed into the transformer 10.

The circuit according to the invention may likewise be used on voltagesources of alternating polarity for rectifying the current and forregulating the power drawn from the voltage source. In this case, at thebeginning of the positive half-cycle, the transformer 10 is connected tothe voltage source by the switching elements 11 and 12 via the seriespath 1 and, during the positive half-cycle, after a time perioddetermined by a suitable regulating device, is again connected to theneutral conductor of the voltage source, series path 2, in order togenerate a positive rectified current for the load. At the beginning ofthe negative half-cycle, the transformer 10 is connected to the voltagesource by the switching elements 11 and 12 via the series path 1 and,during the negative half-cycle, after a time period determined by asuitable regulating device, is again connected to the neutral conductorof the voltage source, series path 2, in order to generate a negativerectified current for the load.

The circuit according to the invention may furthermore be used onvoltage sources of alternating polarity with a neutral conductor, suchas, for example, the secondary winding of a transformer with a centertap, for rectifying the current and for regulating the power drawn fromthe voltage source. In this case, at the beginning of the positivehalf-cycle, the transformer 10 remains connected by the switchingelements 11 and 12 to the—at this point in time—positive terminal of thevoltage source, and, during the positive half-cycle, at a switchinginstant determined by a suitable regulating device, is connected via theswitching elements 11 and 12 to the—at this point in time—negativeterminal of the voltage source. This connection persists to the end ofthis half-cycle, and through the polarity reversal of the voltage sourceright into the next half-cycle. In this way, by defining the switchinginstant, it is possible for the load to be fed a positive current if theswitching instant lies in the second half of the half-cycle, a negativecurrent if the switching instant lies in the first half of thehalf-cycle, and no current if the switching instant lies in the centerof the half-cycle.

FIG. 6 shows the possibility of how the circuit described in theapplication can be used as an active impedance. It can thereby be usedvery simply as a replacement for an ohmic resistor for current limiting.In this case, the circuit is completely encapsulated and, like a simpleimpedance, provided only with two terminals. One path is thus notconnected.

The circuit may acquire an ohmic characteristic, be embodied as avoltage source or current source, or operate with an additional controlinput as a potentiometer or power controller, an extremely low powerloss occurring in each case.

In the case of the circuit illustrated in FIG. 6 b, the power lossamounts to only approximately 0.05*U*I, that is to say approximately 5percent of the power loss which would occur at the ohmic resistor ofFIG. 6 a. The value of the impedance is dependent on the value of theinductance L 10 and the capacitance C 13.

Subclaims relate to further refinements of the invention. In this case,such feature combinations for which no express example has beenspecified are also to be regarded as claimed.

1. A circuit arrangement for the reliable switching of electricalcircuits, in a lead to a current load (3), comprising a first seriespath (1, 2), which electrically connects a first input point (4, 5) to afirst output point (6, 7), and a second series path (1, 2), whichelectrically connects a second input point (4, 5) to a second outputpoint (6, 7), characterized in that at least two switching elements (11,12) are arranged in parallel with one another in at least one of theseries paths (1, 2), the switching inputs (20) of said switchingelements being connected to the input point of the series path (4, 5)and the switching outputs (21) of said switching elements beingconnected to the input side of a respective winding (8, 9) of atransformer (transformer 10), and the output sides of the windings (8,9) are connected to one another and to the output point of the seriespath (6, 7), and the windings of the transformer are embodied such thatthe magnetic field strengths (17, 18) of the two component currents (15,16) of the load current (14) in the transformer (transformer 10) actoppositely to one another, and a capacitance (13) is effective betweenthe output points (6, 7) of the series paths (1, 2).
 2. The circuitarrangement as claimed in claim 1, characterized in that a capacitor(19) is arranged in series with the load in at least one of the seriespaths (1, 2).
 3. The circuit arrangement as claimed in claim 1 or 2,characterized in that at least one of the switching elements (11, 12)has a control input (23) which, when actuated, interrupts the connectionbetween the switching input (20) and the switching output (21).
 4. Thecircuit arrangement as claimed in claim 1 or 2, characterized in that atleast one of the switching elements (11, 12) contains a fuse locatedbetween the switching input (20) and the switching output (21).
 5. Thecircuit arrangement as claimed in claim 3, characterized in that atleast one of the switching elements (11, 12) has a further input (22),which is conductively connected to the series path (1, 2)., and containsat least one nonlinear element (24) located between the input (22) andthe switching output (21).
 6. The circuit arrangement as claimed inclaim 5, characterized in that the nonlinear element (24) becomesconductive if the polarity of the voltage at the switching output (21)becomes opposite to the polarity at the switching input (20).
 7. Thecircuit arrangement as claimed in claim 5, characterized in that thenonlinear element (24) becomes conductive if the magnitude of thevoltage at the switching output (21) exceeds the magnitude of thevoltage at the switching input (20).
 8. The circuit arrangement asclaimed in claim 3, characterized in that the control inputs (23) of theswitching elements (11, 12) are actuated virtually simultaneously by asuitable device.
 9. The circuit arrangement as claimed in claim 8,characterized in that the control inputs (23) of the switching elements(11, 12) are connected to one another and are actuated virtuallysimultaneously by at least one suitable device.
 10. The circuitarrangement as claimed in claim 1 or 2, characterized in that at leastone of the switching elements (11, 12) has a control input (23) which,when actuated interrupts the connection between the switching input (20)and switching output (21), and establishes the connection to a furtherswitching input (22), which is conductively connected to the series path(1, 2).
 11. The circuit arrangement as claimed in claim 10,characterized in that the control inputs (23) of the switching elements(11, 12) are connected to one another and are actuated virtuallysimultaneously by at least one suitable device.
 12. The circuitarrangement as claimed in claim 1 or 2, characterized in that at leastone of the switching elements (11, 12) contains a controllable component(25, 29) with a control input (26, 30), which can establish andinterrupt the connection between the switching input (20) and theswitching output (21) of the switching elements.
 13. The circuitarrangement as claimed in claim 1 or 2, characterized in that at leastone of the switching elements (11, 12) contains a controllable component(27, 31) with a control input (28, 32), which can establish andinterrupt the connection between the switching input (22) and theswitching output (21) of the switching elements.
 14. The circuitarrangement as claimed in claim 1 or 2, characterized in that at leastone of the switching elements (11, 12) contains two controllablecomponents (25, 27) and (29, 31) each with a control input (26, 28) and(30, 32), respectively, which can establish and interrupt the connectionbetween the switching inputs (20) and the switching outputs (21) andalso between the switching inputs (22) and the switching outputs (21).15. The circuit arrangement as claimed in claim 12, 13 or 14,characterized in that the controllable component comprises at least onerelay contact.
 16. The circuit arrangement as claimed in claim 12, 13 or14, characterized in that the controllable component comprises at leastone transistor.
 17. The circuit arrangement as claimed in claim 12, 13or 14, characterized in that the controllable component comprises atleast one thyristor.
 18. The circuit arrangement as claimed in claim 14,characterized in that the control inputs (26, 28, 30, 32) of thecomponents (25, 27, 29, 31) are led to a control device (33), which canassume at least two states, state 1 and state 2, and which has at leastone control input (36).
 19. The circuit arrangement as claimed in claim18, characterized in that the control device (33) in state 1 drives thecomponents (25, 29) such that the latter conductively connect theswitching inputs (20) to the switching outputs (21) of the switchingelements (11, 12), and drives the components (27, 31) such that thelatter interrupt the connection between the switching inputs (22) andthe switching outputs (21) of the switching elements (11, 12).
 20. Thecircuit arrangement as claimed in claim 18, characterized in that thecontrol device (33) in state 2 drives the components (25, 29) such thatthe latter interrupt the connections between the switching inputs (20)and the switching outputs (21) of the switching elements (11, 12), anddrives the components (27, 31) such that the latter establish theconductive connection between the switching inputs (22) and theswitching outputs (21) of the switching elements (11, 12).
 21. Thecircuit arrangement as claimed in claims 18 to 20, characterized in thatthe control device (33) is changed over from state 1 to state 2 via thecontrol input (36), and the control device (33), upon this statetransition, drives the components (25, 29) virtually simultaneously suchthat the connection between the switching inputs (20) and the switchingoutputs (21) is interrupted, subsequently drives one of the components(27) or (31) such that the connection between the switching input (22)and switching output (21) thereof is established, and then drives thesecond component (27) or (31) via the control input (28) or (32) thereofsuch that the connection between the switching input (22) and switchingoutput (21) thereof becomes conductive if the voltage between theswitching input (22) and switching output (21) thereof has becomevirtually zero.
 22. The circuit arrangement as claimed in claims 18 to20, characterized in that the control device (33) is changed over fromstate 2 to state 1 via the control input (36), and, upon this statetransition, drives the components (27, 31) virtually simultaneously suchthat the connection between the switching inputs (22) and the switchingoutputs (21) is interrupted, subsequently drives one of the components(25) or (29) such that the connection between the switching input (20)and switching output (21) thereof is established, and then drives thesecond component (25) or (29) via the control input (26) or (30) thereofsuch that the connection between the switching input (20) and switchingoutput (21) thereof becomes conductive if the voltage between theswitching input (20) and switching output (21) thereof has becomevirtually zero.
 23. The circuit arrangement as claimed in claims 18 to20, characterized in that the control device (33) is changed over fromstate 1 to state 2 via the control input (36), and, upon this statetransition, drives the components (25, 29) virtually simultaneously suchthat the connection between the switching inputs (20) and the switchingoutputs (21) is interrupted, subsequently then drives one of thecomponents (27) or (31) such that the connection between the switchinginput (22) and switching output (21) thereof is established if thevoltage between switching input (22) and switching output (21) hasbecome virtually zero, and then drives the second component (27) or (31)via the control input (28) or (32) thereof such that the connectionbetween the switching input (22) and switching output (21) thereofbecomes conductive if the voltage between the switching input (22) andswitching output (21) thereof has become virtually zero.
 24. The circuitarrangement as claimed in claims 18 to 20, characterized in that thecontrol device (33) is changed over from state 2 to state 1 via thecontrol input (36), and, upon this state transition, drives thecomponents (27, 31) virtually simultaneously such that the connectionbetween the switching inputs (22) and the switching outputs (21) isinterrupted, subsequently then drives one of the components (25) or (29)such that the connection between the switching input (20) and switchingoutput (21) thereof is established if the voltage between switchinginput (20) and switching output (21) has become virtually zero, and thendrives the second component (25) or (29) via the control input (26) or(30) thereof such that the connection between the switching input (20)and switching output (21) thereof becomes conductive if the voltagebetween the switching input (20) and switching output (21) thereof hasbecome virtually zero.
 25. The circuit arrangement as claimed in claims18 to 20, characterized in that a voltage source having alternatingpolarity, such as, for example, a mains voltage, the secondary windingof a transformer, the secondary winding of a switched-mode power supplytransformer, is connected to the input points (4, 5).
 26. The circuitarrangement as claimed in claim 25, characterized in that the controldevice (33) has a further control input (34) connected to the seriespath (1).
 27. The circuit arrangement as claimed in claim 25,characterized in that the control device (33) has a further controlinput (35) connected to the series path (2).
 28. The circuit arrangementas claimed in claims 21 to 27, characterized in that the transition fromstate 2 to state 1 in the switching device (33) is triggered by thecontrol input (34) or (35) when the voltage between input point (4) andinput point (5) becomes positive, and the transition from state 1 tostate 2 is triggered by the control input (36).
 29. The circuitarrangement as claimed in claims 21 to 27, characterized in that thetransition from state 2 to state 1 in the switching device (33) istriggered by the control input (34) or (35) when the voltage betweeninput point (4) and input point (5) becomes negative, and the transitionfrom state 1 to state 2 is triggered by the control input (36).
 30. Thecircuit arrangement as claimed in claims 25 to 27, characterized in thata second voltage source having alternating polarity, such as, forexample, a mains voltage, the secondary winding of a transformer, or thesecondary winding of a switched-mode power supply transformer, isconnected to input point (5) and to a further input point (40) such thatthe input point (40) has the opposite polarity to the input point (4).31. The circuit arrangement as claimed in claim 30, characterized inthat the switching elements (11, 12) in each case have a furtherswitching input (41), which is connected to the input point (40) via afurther series path (42), and in each case have at least one furthercontrollable component (43, 44) with the control inputs (45, 46), whichcontrollable component can be driven such that it establishes aconductive connection between switching input (41) and switching output(21).
 32. The circuit arrangement as claimed in claim 31, characterizedin that the control device (33) is connected to the control input (45)of the component (43) via a further control output (47), to the controlinput (46) of the component (44) via a further control output (48) andto the series path (42) via a further input (49).
 33. The circuitarrangement as claimed in claim 32, characterized in that the controldevice (33), during the rise of the input voltage (4, 5), drives thecomponents (25, 27, 29, 31) such that the switching inputs (20) areconductively connected to the switching outputs (21) and the conductiveconnection between the switching inputs (22) and the switching outputs(21) is interrupted, and, during the rise of the input voltage (40, 5),drives the components (27, 31, 43, 44) such that the switching inputs(41) are conductively connected to the switching outputs (21), and theconductive connection between the switching inputs (22) and theswitching outputs (21) is interrupted, and, by means of a suitablesignal of the control input (36) of the control device (33), thecomponents (25, 29, 43, 44) are driven such that the conductiveconnections between switching inputs. (20, 41) and the switching output(21) are interrupted, and the components (27, 31) are driven such that aconductive connection is produced between the switching inputs (22) andswitching outputs (21).
 34. The circuit arrangement as claimed in claim32, characterized in that the control device (33), during the fall ofthe input voltage (4, 5), drives the components (25, 27, 29, 31) suchthat the switching inputs (20) are conductively connected to theswitching outputs (21) and the conductive connection between theswitching inputs (22) and the switching outputs (21) is interrupted,and, during the fall of the input voltage (40, 5), drives the components(27, 31, 43, 44) such that the switching inputs (41) are conductivelyconnected to the switching outputs (21), and the conductive connectionbetween the switching inputs (22) and the switching outputs (21) isinterrupted, and, by means of a suitable signal of the control input(36) of the control device (33), the components (25, 29, 43, 44) aredriven such that the conductive connections between switching inputs(20, 41) and the switching output (21) are interrupted, and thecomponents (27, 31) are driven such that a conductive connection isproduced between the switching inputs (22) and switching outputs (21).35. The circuit arrangement as claimed in claim 32, characterized inthat the control device (33), during the rise of the input voltage (4,5), drives the components (25, 27, 29, 31) such that the switchinginputs (20) are conductively connected to the switching outputs (21)when the voltage measured at them is virtually zero, and the conductiveconnection between the switching inputs (22) and the switching outputs(21) is interrupted, and, during the rise of the input voltage (40, 5),drives the components (27, 31, 43, 44) such that the switching inputs(41) are conductively connected to the switching outputs (21) when thevoltage measured at them is virtually zero, and the conductiveconnection between the switching inputs (22) and the switching outputs(21) is interrupted, and, by means of a suitable signal at the controlinput (36) of the control device (33), the components (25, 29, 43, 44)are driven such that the conductive connections between switching inputs(2041) and the switching output (21) are interrupted, and the components(27, 31) are then driven such that a conductive connection is producedbetween the switching inputs (22) and switching outputs (21) if thevoltage measured at them is virtually zero.
 36. The circuit arrangementas claimed in claim 32, characterized in that the control device (33),during the fall of the input voltage (4, 5), drives the components (25,27, 29, 31) such that the switching inputs (20) are conductivelyconnected to the switching outputs (21) when the voltage at them isvirtually zero, and the conductive connection between the switchinginputs (22) and the switching outputs (21) is interrupted, and, duringthe fall of the input voltage (40, 5), drives the components (27, 31,43, 44) such that the switching inputs (41) are conductively connectedto the switching outputs (21) when the voltage at them is virtuallyzero, and the conductive connection between the switching inputs (22)and the switching outputs (21) is interrupted, and, by means of asuitable signal at the control input (36) of the control device (33),the components (25, 29, 43, 44) are driven such that the conductiveconnections between switching inputs (20, 41) and the switching output(21) are interrupted, and the components (27, 31) are then driven suchthat a conductive connection is produced between the switching inputs(22) and switching outputs (21) if the voltage measured at them isvirtually zero.
 37. The circuit arrangement as claimed in claims 18 to20, characterized in that there is connected between input point (4) anda further input point (50) a voltage source having alternating polarity,such as, for example, the secondary winding of a transformer, which hasan additional tapping for voltage division, such as, for example, acenter tap of the secondary winding of the transformer, and saidadditional tapping is connected to the input point (5) of the circuitarrangement as claimed in the invention.
 38. The circuit arrangement asclaimed in claims 25, 26 and 37, characterized in that the input point(50) is connected by a further series path (51) to the switching inputs(22) of the switching elements (11, 12) and to the input (35) of thecontrol device (33).
 39. The circuit arrangement as claimed in claim 38,characterized in that the control device (33), by means of a suitablesignal at its control input (36), is switched to state 1 during thefirst half-cycle of the input voltage at the input points (4, 50) and tostate 2 during the second half-cycle of the input voltage, and therespectively existing state 1 or 2 is maintained during the zerocrossing of the input voltage.
 40. The circuit arrangement as claimed inclaims 19 and 20, characterized in that the control device (33), uponthe transition from state 2 to state 1, then drives the components (25,29) such that they establish a conductive connection between switchinginput (20) and switching output (21) if the voltage between switchinginput (2.0) and switching output (21) has become virtually zero.
 41. Thecircuit arrangement as claimed in claims 19 and 20, characterized inthat the control device (33), upon the transition from state 1 to state2, then drives the components (27, 31) such that they establish aconductive connection between switching input (22) and switching output(21) if the voltage between switching input (22) and switching output(21) has become virtually zero.
 42. The circuit arrangement as claimedin claims 18, 32 and 38, characterized in that the control device (33)is embodied as an integrated circuit.
 43. The circuit arrangement asclaimed in claims 18, 32 and 38, characterized in that the controldevice (33) is embodied as an electronic assembly.
 44. The circuitarrangement as claimed in claims 18, 32 and 38, characterized in thatthe control device (33) is embodied as a potted module.
 45. The circuitarrangement as claimed in claims 18, 31, 32 and 38, characterized inthat the control device (33) together with the controllable components(25, 27, 29, 31, 43, 44) is embodied as an integrated circuit.
 46. Thecircuit arrangement as claimed in claims 18, 31, 32 and 38,characterized in that the control device (33) together with thecontrollable components (25, 27, 29, 31, 43, 44) is embodied as anelectronic assembly, hybrid circuit, thick-film circuit or thin-filmcircuit.
 47. The circuit arrangement as claimed in claims 18, 31, 32 and38, characterized in that the control device (33) together with thecontrollable components (25, 27, 29, 31, 43, 44) is embodied as a pottedmodule or module encapsulated by a housing.
 48. The circuit arrangementas claimed in claim 1, characterized in that the two windings (8, 9) ofthe transformer (10) are embodied as two parallel-routed lines orconductor tracks of a printed circuit, which are connected to theswitching elements (11, 12) and the output point (6) of the circuitarrangement as claimed in the invention such that the component currents(15, 16) of the load current (14) flow through them in an oppositedirection.
 49. The circuit arrangement as claimed in claim 1,characterized in that the capacitance (13) is formed from twoparallel-routed lines or conductor tracks of a printed circuit.